Strategies to reduce carbon dioxide in the atmosphere have turned attention to the ocean. Among the most explored proposals are marine carbon removal methods (mCDR), designed to enhance CO₂ absorption by aquatic ecosystems. However, new research has warned about adverse effects of these interventions.
Several biological approaches to mCDR, far from being harmless solutions, could worsen an already critical issue: ocean oxygen loss. This phenomenon, known as deoxygenation, threatens marine life. In recent decades, global warming has reduced 2% of the oxygen available in the seas, altering habitats and species.
The study, based on global simulations, evaluated multiple techniques. Some, such as ocean fertilization or sinking algae biomass, proved to be particularly risky. These processes promote the growth of photosynthetic organisms whose decomposition later consumes large amounts of oxygen. Thus, what is gained by mitigating climate change is lost in terms of marine health.
Algae, Oxygen, and Invisible Consequences
One of the most critical methods is sinking macroalgae after large-scale cultivation. Although initially seeming like a natural strategy, its degradation in deep areas depletes oxygen at alarming levels. Simulations showed that this type of mCDR could lead to a drop in oxygen between 4 and 40 times greater than the benefits of reducing global warming.
Artificial upwelling —a process that brings up deep nutrient-rich waters— also poses a danger. While it boosts temporary biological productivity, it accelerates decomposition processes that worsen ocean deoxygenation, a phenomenon already affecting especially sensitive species.
In contrast to these approaches, geochemical methods, such as increasing seawater alkalinity with limestone compounds, showed less ecological impact. As they do not involve nutrient input or organic decomposition, they do not generate significant oxygen loss and resemble, in their effects, a simple emissions reduction.
Cultivating Algae as an Exception for Ocean Life Preservation
A safer alternative is macroalgae cultivation with biomass harvesting. Unlike sinking, this technique allows CO₂ absorption without consuming marine oxygen, as nutrients are removed with the biomass before its decomposition in the ocean.
Even large-scale models suggest that this strategy could reverse part of the oxygen loss already experienced. However, extracting marine nutrients could negatively impact other forms of biological productivity. Therefore, its implementation would require careful balance between carbon absorption and ecological conservation.
The ocean is a complex and fragile system, where each intervention can have collateral consequences. In this context, scientists insist that any proposal for marine carbon removal must include, as a mandatory aspect, continuous monitoring of oxygen levels.
Urgency and Caution with New Technologies
While climate policies progress, the need to achieve net zero emissions has led to considering more radical options such as mCDR. Although the ocean is a great ally in natural carbon capture, massively intervening in its processes poses risks not yet fully understood.
The race to curb climate change cannot come at the expense of marine ecosystem health. Well-intentioned measures, if not rigorously evaluated, could end up worsening environmental degradation. Preservation of oceanic oxygen must become a key focus in the planning of any future climate action.
The Importance of Carbon Dioxide in Ocean Life
Carbon dioxide (CO₂) plays an essential role in ocean life, being the basis of the photosynthesis process carried out by phytoplankton, algae, and other marine organisms. These microscopic life forms, by capturing CO₂ and releasing oxygen, not only sustain the marine food chain but also contribute to producing a large part of the oxygen we breathe on Earth.
Additionally, CO₂ dissolved in the ocean participates in the water’s chemical balance, helping to regulate its acidity. This balance is vital for the formation of shells and skeletons of many marine organisms, such as corals, mollusks, and certain types of calcareous plankton, whose existence depends on calcium carbonate.
However, an excess of CO₂ —a product of human emissions— can disrupt this delicate balance, leading to ocean acidification. This phenomenon threatens entire ecosystems and compromises the survival of key species, showing that while CO₂ is vital for marine life, its concentration must be kept within natural limits.
